The present invention relates to the manufacture of high quality graphite electrodes for use in electric arc furnaces.
Graphite electrodes for use in electric arc furnaces are conventionally made by shaping a mixture containing a carbonaceous filler material such as petroleum coke and a binder such as coal tar pitch to the form of the desired electrode and then baking the electrode at high carbonizing temperatures. The electrode is then graphitized at elevated temperatures of at least 2500.degree. C.
The quality of graphite electrodes so produced is mainly dependent upon the type of raw materials that are employed. Graphite electrodes capable of superior performance can usually be made by incorporating into the electrode mixture a premium petroleum coke as a filler material. These electrodes exhibit a low electrical resistivity and a low coefficient of thermal expansion (CTE) and also possess a high flexural strength. Unfortunately, the cost of manufacturing these electrodes has risen dramatically over the past few years due mainly to the increased cost and at times reduced availability of premium petroleum cokes. There is therefore a growing need in the industry to develop improved methods for manufacturing graphite electrodes using lower cost and more plentiful raw materials.
It has already been proposed to replace part of the premium petroleum coke used in the manufacture of high quality graphite electrodes with lower cost carbonaceous materials such as, for example, regular petroleum cokes, metallurgical coke or previously baked or graphitized carbonaceous materials. However, these proposals have not met with satisfactory results primarily because electrodes made with these substitute materials exhibit high CTE values and, in some cases, a low flexural strength.
Although coal particles have been occasionally found in minute amounts in graphite electrodes, they have been regarded as an undesirable contaminant and, consequently, the use of coal in any amounts as a raw material in the manufacture of graphite electrodes has been avoided. Coal flours of less than 100 mesh size have been evaluated experimentally as a raw material but the test results indicated intolerable CTE values of two to four times that obtained for premium cokes.